The present invention relates to a filter of a high accuracy. More specifically, it relates to a filter having a high accuracy, small initial pressure loss, and long filtration life compared with conventional filters.
In recent years, a demand has suddenly increased for filters comprising fibers as their material for removing foreign substances having a particle diameter of about 0.2 xcexcm to several hundred micro-meters contained in solutions of products or others in the field of chemical industry.
Filters comprising fibers as their material have various characteristics such that they are generally inexpensive, that filtration accuracy can unrestrictedly be changed by changing the diameter of fibers from which filters are formed, and that they have a long filtration life since they can capture many particles in void portions between fibers. However, filters having a higher filtration accuracy and longer filtration life than conventional ones have strongly been demanded in some end uses of filters.
Performances of such filters can be evaluated mainly by filtration accuracy, liquid passability, and filtration life. The term xe2x80x9cfiltration accuracyxe2x80x9d as used herein is represented by a smallest diameter of particles which can be captured 99.9% or more, the term xe2x80x9cliquid passabilityxe2x80x9d is represented by a pressure loss of a filter when a certain amount of liquid was flowed, and the term xe2x80x9cfiltration lifexe2x80x9d is represented by an elapsed time until the pressure loss in a filter generated by a continuous filtration of a dispersion having a certain concentration reaches a value at which renewal of the filter becomes necessary.
Generally, the higher the filtration accuracy of filters, the lower liquid passability and the shorter filtration life the filters have in many cases. However, even with filters of a high filtration accuracy, the liquid passability and filtration life can be improved by making the structure of filter not-uniform in terms of the type of materials or mediums to be used and changing diameter of fibers in a filter and void ratio in the direction of filtration, that is, from the upstream side of a liquid to be passed through the filter toward the downstream side. Heretofore, several methods have been proposed for improving such liquid passability and filtration life.
First, a method is disclosed in Laid-open Japanese Patent Publication No. Hei 7-82649 wherein the amount of a resin to be extruded and the flow velocity of a blowing gas stream are changed with the passage of time in a fiber spinning step by a melt-blow process to continuously change the average diameter of fibers to be spun, and then the fibers are wound round an appropriate winding core. Whereas this method has a merit that the average diameter of fibers which form the filters can comparatively be freely changed, it has a demerit that there is a limit in the range wherein either the amount of a resin to be extruded and flow velocity of a blowing gas stream can be changed and thus that it is difficult to largely change the fiber diameter in the direction of filter thickness (direction of filtration). Besides, it has a defect that it is difficult to produce fine denier fibers since spinning conditions are continuously changed in the method.
In Laid-open Japanese Patent Publication No. Hei 1-297113, a method is disclosed in which several kind of non-woven fabrics having a different fiber diameter are prepared in advance and then the non-woven fabrics are wound round a core having holes to prepare a filter. According to this method, it is possible to largely change the fiber diameter. However, the method has defects that a number of the non-woven fabrics become necessary in order to efficiently perform deep layer filtration, because gentle change of fiber diameter in the direction of the filter thickness is required to achieve the performance, and thus the number of production steps increases and requires much time.
Further, in Japanese Patent Publication No. Sho 56-49605, a method is disclosed wherein a card web of melt adhesive staple fibers is wound round a winding core while heating and applying an appropriate linear pressure on the web, and a melt-blow non-woven fabric is wound together on the way of winding the card web to form a filter. This method has a characteristic that filters having such an extent of accuracy as that of filters prepared by a melt-blow process can be produced while taking advantage of using cheap staple fibers. However, the method has a defect that when the diameter of particles to be captured is extremely smaller than the fiber diameter of staple fibers, small particles are captured only in the inserted melt-blow non-woven fabric and thus the filtration life becomes short.
An object of the present invention is to provide filters having such a high accuracy and long filtration life as those which were unable to be achieved by conventional filters, at a low production cost.
As a result of diligent investigation by the present inventors to solve the problems described above, it has been found that the objects of the present invention can be achieved by disposing at least two layers of a pre-filtration layer and a precision filtration layer in a filter comprising non-woven fibrous agglomerates, constructing the pre-filtration layer so that the diameter of constituting fibers becomes finer in the direction of filtration, and constructing the precision filtration layer with a non-woven fibrous agglomerate comprising fibers having a smaller diameter than that of the fibers having a smallest diameter in the pre-filtration layer to accomplish the present invention.
The present invention is summarized as follows:
(1) A filter of a high accuracy composed of non-woven fibrous agglomerates and comprising at least two layers of a pre-filtration layer and a precision filtration layer disposed in the direction of filtration, the pre-filtration layer is constructed in such a way that the diameter of all or a part of the fibers in the pre-filtration layer becomes gradually smaller toward the direction of filtration, the precision filtration layer comprises one or more piled layers of a non-woven fibrous agglomerate, and the diameter of fibers which account for 10% by weight or more of the fibers in the one or more piled layers of a non-woven fibrous agglomerates is smaller than the diameter of the fibers having a smallest diameter in the pre-filtration layer.
(2) The filter of a high accuracy recited in paragraph (1) above wherein the pre-filtration layer comprises a non-woven fibrous agglomerate comprising at least one kind of fibers selected from the group consisting of polyolefin fibers and polyester fibers.
(3) The filter of a high accuracy recited in paragraph (1) or (2) above wherein the pre-filtration layer comprises a non-woven fibrous agglomerate prepared by a melt-blow process.
(4) The filter of a high accuracy recited in any one of paragraphs (1) to (3) above wherein the fibers in the non-woven fibrous agglomerate of the pre-filtration layer comprise a high melting point component and a low melting point component, the difference in melting point between the high melting point component and the low melting point component is 10xc2x0 C. or more, and the low melting point component is contained in an amount of 10 to 90% by weight in the pre-filtration layer.
(5) The filter of a high accuracy recited in paragraph (4) above wherein the fibers of the non-woven fibrous agglomerate of the pre-filtration layer are composite fibers comprising a high melting point component and a low melting point component having a difference in melting point of 10xc2x0 C. or more.
(6) The filter of a high accuracy recited in paragraph (4) above wherein the non-woven fibrous agglomerate of the pre-filtration layer is a mixture of ultrafine fibers of a high melting point component and ultrafine fibers of a low melting point component having a difference in melting point of 10xc2x0 C. or more.
(7) The filter of a high accuracy recited in any one of paragraphs (1) to (6) above wherein the ratio of a smallest diameter to a largest diameter of fibers in the non-woven fibrous agglomerate of the pre-filtration layer is 1:2 to 1:10.
(8) The filter of a high accuracy recited in any one of paragraphs (1) to (7) above wherein the ratio of the diameter of fibers in the non-woven fibrous agglomerate of the precision filtration layer which comprises fibers having a diameter smaller than that of fibers having a smallest diameter in the pre-filtration layer to a smallest diameter of the fibers in the pre-filtration layer is 1:1 to 1:20.
(9) The filter of a high accuracy recited in any one of paragraphs (1) to (8) above wherein the non-woven fibrous agglomerate of the precision filtration layer which comprises fibers having a diameter smaller than that of the fibers having a smallest diameter in the pre-filtration layer has a void ratio of 45 to 97%.
(10) The filter of a high accuracy recited in any one of paragraphs (1) to (9) above wherein the non-woven fibrous agglomerate of the precision filtration layer which comprises fibers having a diameter smaller than that of the fibers having a smallest diameter in the pre-filtration layer is prepared by a melt-blow process.
(11) The filter of a high accuracy recited in any one of paragraphs (1) to (9) above wherein the non-woven fibrous agglomerate of the precision filtration layer which comprises fibers having a diameter smaller than that of the fibers having a smallest diameter in the pre-filtration layer comprises glass fibers.
(12) The filter of a high accuracy recited in any one of paragraphs (1) to (11) above wherein the filter is in a shape of cylinder.